In recent years, optical breakdown induced in transparent materials by femtosecond laser pulses, and its application to micromachining, have received much attention. As to the former, optical breakdown induced in transparent materials by femtosecond laser pulses, reference may be had, for example, to an article by Du et al. entitled “Laser-Induced Breakdown by Impact Ionization in SiO2 with Pulse Widths from 7 ns to 150 fs,” appearing at Appl. Phys. Lett. 64, 3071 (1994), to an article by Stuart et al. entitled “Optical Ablation by High-Power Short-Pulse Lasers,” appearing at J. Opt. Soc. Am. B 13, 459 (1996), to an article by Lenzner et al. entitled “Femtosecond Optical Breakdown in Dielectrics,” appearing at Phys. Rev. Lett. 80, 4076 (1998), and to an article by Schaffer, et al. entitled “Laser-Induced Breakdown and Damage in Bulk Transparent Materials Induced by Tightly-Focused Femtosecond Laser Pulses,” appearing at Meas. Sci. Technol. 12, 1784 (2001), each incorporated herein by reference. As to the latter, the application to micromachining, reference may be had, for example, to an article by Varel et al. entitled “Micromachining Quartz with Ultrashort Laser Pulses,” appearing at Appl. Phys. A. 65, 367 (1997), an article by Glezer et al. entitled “Three-Dimensional Optical Storage Inside Transparent Materials,” appearing at Opt. Lett. 21, 2023 (1996), to another article by Glezer et al. entitled “Ultrafast-Laser Driven Micro-Explosions in Transparent Materials,” appearing at Appl. Phys. Lett. 71, 882 (1997), to an article by Schaffer et al., entitled “Laser Induced Microexplosions in Transparent Materials: Microstructuring with Nanojoules,” appearing at Proc. SPIE 3616, 143 (1999), to an article also by Schaffer et al. entitled “Microscopic Bulk Damage in Dielectric Materials Using Nanojoule Femtosecond Laser Pulses,” appearing at OSA Technical Digest: Quantum Electronics and Laser Science Conference 99, 232 (1999), to an article by Davis et al. entitled “Writing Waveguides in Glass with a Femtosecond Laser,” appearing at Opt. Lett. 21, 1729 (1996), to an article by Miura et al. entitled “Photowritten Optical Waveguides in Various Glasses with Ultrashort Pulse Laser,” appearing at Appl. Phys. Lett. 71, 3329 (1997), to another article by Schaffer et al. entitled “Micromachining Optical Waveguides in Bulk Glass Using a Femtosecond Laser Oscillator,” appearing at OSA Technical Digest: Conference on Lasers and Electro Optics 375 (2000), to an article by Homoelle et al. entitled “Infrared Photosensitivity in Silica Glasses Exposed to Femtosecond Laser Pulses,” appearing at Opt. Lett. 24, 1311 (1999), to another article by Schaffer, et al. entitled “Micromachining Bulk Glass by Use of Femtosecond Laser Pulses with Nanojoule Energy,” appearing at Opt. Lett. 26, 93 (2000), and to an article by Minoshima, et al. entitled “Photonic Device Fabrication in Glass by Use of Nonlinear Materials Processing with a Femtosecond Laser Oscillator,” appearing at Opt. Lett. 26, 1516 (2001), each incorporated herein by reference.
When a femtosecond laser pulse is focused inside the bulk of a transparent material, the intensity in the focal volume can become high enough to cause absorption through non-linear processes, leading to optical breakdown in the material. See, in this connection, the articles by Du et al., Stuart et al. and Lenzner et al., supra. Because the absorption is strongly non-linear, this breakdown can be localized in the focal volume inside the bulk of the material, leaving the surface unaffected. See, in this connection, the two articles by Glezer et al. and the first three articles by Schaffer et al., supra. The energy deposited in the material produces permanent structural changes in the sample, which can be used to micromachine a three-dimensional (3-D) structure inside the bulk of the sample. See, in this connection, for example, the article by Stuart et al., the two articles by Glezer et al. and the articles by Schaffer et al., supra. Recent demonstrations have included three-dimensional binary data storage as reported by the first Glezer et al. article, and the direct writing of optical waveguides and waveguide splitters as respectively reported by the Davis et al., Miura et al. and the fourth and fifth listed Schaffer et al. articles, supra, and by the Homoelle et al article, supra.
Until now, however, micromachining with femtosecond laser pulses required amplified laser systems that not only were expensive, sometimes unreliable and generally complex, but also those amplified laser systems, operating at kilohertz repetition-rates, severely limited the maximum processing speed for many applications.
In addition, the utility of bulk waveguide laser micromachining techniques has heretofore been limited to fabrication of waveguides or other devices of generally circular cross-section that lie along a geometric straight line in the bulk. Since a waveguide must have a transverse spatial extent of at least several microns to efficiently guide visible or near infrared light, the heretofore known bulk waveguide laser micromachining techniques have generally employed focusing optics with numerical apertures (NA) below 0.25 to produce a focused spot size larger than the several microns necessary in order to fabricate waveguides. See, for example, the article by Homoelle et al., supra. But because the focal volume is significantly elongated in the axial direction with the numerical apertures needed to produce spot sizes larger than the several microns necessary for waveguides, it was heretofore necessary to write the waveguides axially, as transverse linear writing resulted in a waveguide with an elliptical cross-section while transverse non-linear writing resulted in photonic structures of generally nonuniform cross-section. This axial writing, however, limits the writing of waveguides or other devices of generally circular cross-section to structures that lie along a geometric straight line in the bulk, or have very little curvature away from a geometric straight line in the bulk.